High-loading Ga-exchanged MFI zeolites as selective and coke-resistant catalysts for nonoxidative ethane dehydrogenation

Abstract

In this paper, we investigated the effects of the Ga loading amount and H₂ treatment temperature for the reductive solid-state ion-exchange reaction on the generated Ga species in Ga-exchanged MFI zeolites (Ga-MFIs) as well as their catalysis for ethane dehydrogenation (EDH). For the formation of isolated Ga hydrides in the zeolites, [GaH]²⁺ ions were preferentially formed in the low-loading Ga-MFI (Ga/Al = 0.3) treated with H₂ at 550 °C, corresponding to the conventional preparation conditions, (Ga-MFI-0.3(550)), while the high Ga loading (Ga/Al = 1.0) and high-temperature H₂ treatment (800 °C) (Ga-MFI-1.0(800)) induced the formation of [GaH₂]⁺ ions as the major Ga hydrides, as revealed by in situ Fourier transform infrared spectroscopy including the isotope experiment using D₂. In the context of other Ga species, such as Ga⁺ cations and partially reduced Ga oxides (GaO_X), Ga⁺ cations and GaO_X coexist in Ga-MFI-0.3(550), as indicated by pyridine adsorption experiments. On the other hand, GaO_X was hardly observed and a larger amount of Ga⁺ cations was formed in Ga-MFI-1.0(800). The remaining Brønsted acid sites (BASs) were also characterized by the NH₃ adsorption experiment. In the EDH reaction, Ga-MFI-1.0(800) exhibited high selectivity owing to low coke formation, resulting in the highest durability among the series of Ga-MFIs tested. Under the optimized conditions, Ga-MFI-1.0(800) exhibited the highest C₂H₄ formation rate among previously reported Pt-free catalysts. Based on the combined results of characterization, catalyst tests, and kinetic studies, the high selectivity and durability of Ga-MFI-1.0(800) can be ascribed to the low amount of the remaining BASs by isolated Ga species ([GaH]²⁺, [GaH₂]⁺ ions and Ga⁺ cations) as well as the major formation of [GaH₂]⁺ ions among isolated Ga hydrides.